PROJECT SUMMARY Childhood obesity is a national epidemic that disproportionately burdens low income and ethnic minority communities. By preschool, nearly one-third of low income children are already overweight or obese, setting the stage for a myriad of adverse health conditions in adolescence and adulthood. Experts agree that effective, scalable, and sustainable obesity prevention efforts are urgently needed and must shift their focus to early childhood. Preschools offer a very promising environment for implementation and wide dissemination of interventions targeting large numbers of young children. However, limited rigorous implementation research targeting low income and ethnic minority preschool age children has left a major evidence gap, severely constraining the tools available to educators and policymakers to address the obesity epidemic. Individually- focused and single-component interventions have generally failed to produce long-term changes in obesity- related outcomes. Evidence increasingly supports creating entire environments (e.g., preschools) where the healthy choice is the default option. Therefore, we propose to conduct a pragmatic cluster randomized trial, based on our Healthy-By- Default framework, to evaluate the effectiveness and implementation of a multi-level, multi-component intervention that integrates healthy eating and physical activity into preschool routines. Our intervention will involve 1) establishment of organization-level policies to support healthy nutrition and physical activity, 2) integration of nutrition and physical activity into the preschool curriculum, 3) modeling of healthy behaviors by preschool teachers and staff, and 4) engagement of parents as partners in this effort. The study is a collaboration between UCLA and the Child Care Resource Center (CCRC), a large non-profit organization dedicated to ensuring that all children receive high quality, comprehensive preschool/child care experiences, particularly in low resource, diverse communities. We will randomly assign 60 preschools served by CCRC, in underserved areas of Los Angeles County, to the intervention or a usual practice, wait-list control condition. Our aims are to: 1) implement a multilevel obesity prevention intervention for preschool settings that builds on existing evidence-based approaches, stakeholder input, and our pilot research; 2) evaluate the effectiveness of the intervention on child BMI z-scores (primary outcome), and parent-reported child nutrition and physical activity (secondary outcomes); and 3) systematically assess intervention reach, adoption, implementation, adaptation and maintenance, guided by the RE-AIM Framework, which we have tailored for our specific study focus and setting. By simultaneously evaluating both effectiveness and implementation of our intervention, we will efficiently generate new knowledge vital to informing widespread dissemination. Given the staggering long-term societal costs of childhood obesity, rigorous studies to identify effective and scalable obesity prevention interventions are needed now.